1. Field of the Invention
This invention relates to the field of automatic assembly of electronic devices. In particular, the invention relates to counting electronic components on a component tape and printing a sequential count on the cover tape.
2. Description of the Related Art
Companies in the electronics manufacturing services (EMS) industry are constantly presented with the need to determine the inventory of parts currently available for production of complicated machinery and electrical devices and printed circuit board assemblies (PCBAs). Miniaturization of electronic circuits and parts in the electronics and electrical-mechanical arts has evolved to the point where many of the parts used in devices such as calculators, computers, pagers, telephones, and watches are difficult to store and handle when the parts are in inventory or are being used in conjunction with a mass production line utilizing a large supply of components.
Many of the electrical parts, as an example, are so tiny that they are stored on tapes, which are then rolled onto supply reels for handling. The electrical parts may be encapsulated on the surface of a storage tape by providing a cavity on the storage tape over which another tape is placed in order to capsulate or capture the tiny part and protect the parts from damage or destruction. The component dispensing tape is usually provided in reel form, with a single reel having many feet of tape. Depending on the density of component carriers in the tape, such a reel may contain thousands of components.
Along one side of the typical component dispensing tape are tractor drive or index holes. The automated assembly equipment generally includes a toothed drive wheel that advances the tape by engagement with these holes. The spacing between the holes corresponds to the spacing between the carrier compartments so that when the tape is advanced a particular predetermined number of holes, a number of components are advanced, usually to a robotic assembly arm for placement on a circuit board.
In order to better manage component supply and demand, it is desirable to provide each assembly station with only the number of components that are expected to be used in a certain product run or in a certain period of time. As many of the components are individually quite valuable, a manufacturer must keep a running account of the number of such parts in inventory for tracking material flow, reducing waste, and preventing disruptions in daily production line operation. The physical problem of handling the total number of such small parts is extremely difficult from a bulk handling point of view. As a result, the encapsulating storage tapes become a useful method for maintaining these parts in inventory.
The component tapes protect the components and provide a supply reel of sufficient size for handling by employees. On the other hand, such storage complicates the problem of counting parts for the purpose of identifying a total number of such parts in inventory. Furthermore, the small size of these parts becomes a burden when the parts are placed on a manufacturing line where they are used at a rapid pace. Also, it is imperative that the assembly operation maintain a running inventory of parts available to the manufacturing process at all times. Many electronic units contain literally thousands of tiny parts for inclusion in a single finished electrical unit. If one of these parts is exhausted during the manufacturing process, then the entire manufacturing line may be shut down until the exhausted parts can be replaced. Consequently, a manufacturer is burdened with the problem of keeping a running inventory of the number of parts available during the continuous manufacturing of larger electrical units.
A variety of methods are used to track the storage and manufacturing inventory of small parts, including weight, component tape measuring, and component counting methods. Using the weight method, some inventory systems rely on weight as a way of measuring the total number of small items in inventory. Each unit has a known weight, which can be used to calculate the number of units from a total weight of a bulk container of such small units. The disadvantage of this weight determination system is that expensive electronic scales must be used to make the weight checks. Furthermore, these electronic scales are subject to variations in weight measured and therefore must be constantly recalibrated. Also, weight counting systems are not particularly useful or efficient in a continuous manufacturing process. Some of these weight systems have variations caused by environmental conditions, for example heat and dust, and packaging variables which tend to result in erroneous weight calculations, thereby resulting in an erroneous calculation as to the number of tiny parts in inventory.
In component tape measuring methods, the length of component tape needed to supply a desired number of components to an assembly station may be determined manually. Since counting each of the desired number of components is impractical, the number of components in a short length of the tape is counted instead. The length of tape required for a particular assembly run is then estimated based on the component density of the sampled length of tape, taking into account the occurrences of empty carrier compartments in the sampled tape. The desired length of tape is then unrolled, measured and cut. Although somewhat wasteful of components, the tape is generally cut slightly longer than the computed length to ensure that the equipment does not prematurely exhaust the component supply before the corresponding assembly run is completed.
Component counting includes three typical methods for counting reel quantities of electronic components: reel count, gauge, and estimation methods. The accuracy of these methods varies, but they each have numerous disadvantages.
The reel count method, while having an approximate accuracy of 99%, is slow and has a risk of higher attrition. Reel count also exhibits a lack of counters while being immobile. Furthermore, it is time consuming, requires a great deal of training in operation, and is highly susceptible to human setup error.
The disadvantages of the gauge method are that, while there are over 50 possible variants, not every reel type is covered. This method also requires training to operate accurately and is subject to human error in the identity of the reel type, packaging type, and correct components. The gauge method is also subject to estimation error.
As the most commonly used method of counting reel quantities, the estimation method is very inaccurate, even with training. Human error inheres in the inconsistent visual clues. Moreover, the estimation error trend is to underestimate the quantities on almost full reels while overestimating the quantities on almost empty reels.
The adverse effects of the typical counting and tracking methods are numerous. For example, material flow cannot be accurately tracked. This results in backflushing the quantity of material assumed used in production. As production often consumes only part of a reel, this means a significant amount of stock is wasted. Furthermore, an estimated quantity of attrition called autoscrap is added in an attempt to keep the stock accurate. Work in progress (WIP) cycle counting is then used to verify or correct the stock data. The typical error of over one million parts in WIP indicates that this stock check technique is slow and inaccurate. A typical WIP cycle count has an absolute accuracy level of 30%, meaning that the post-hoc material tracking process is very time consuming and, consequently, ineffective. As planning, purchasing, manufacturing, and customer commitment decisions are based on the material integrity, they are adversely affected. Furthermore, poor data integrity causes disruptions in daily production line operation and results in slow kitting times. Consequently, there is a need for a method and apparatus for determining and printing an electronic component count on the component tape and using the count information to increase the efficiency of the production process.